PROJECT SUMMARY/ABSTRACT Germinal centers (GCs) are site of antibody somatic hypermutation (SHM) and affinity maturation, and are therefore essential for effective humoral immune responses to pathogens. Efficient affinity maturation in GCs requires that B cells cycle between two different cell states, associated with distinct microanacomical compartments: (i) a ?proliferation? mode, when B cells divide and undergo SHM, which takes place in the dark zone (DZ) of the GC; and (ii) a ?selection? mode, when mutated B cells compete with each other based on their affinity for antigen, which takes place in the GC light zone (LZ). While our previous work defined the transcriptional programs broadly associated with LZ and DZ states, our understanding of how cells decide when to transition between one state and another remains incomplete. For example, we do not understand the signals that trigger DZ B cells to stop proliferating and return to the LZ, nor do we know whether LZ and DZ cells can be further parsed into additional transcriptionally distinct states that associated with different stages of GC selection. Here, we propose to combine in vivo models of synchronized GC selection with novel mouse genetic models, treatment with signaling inhibitors, and single-cell mRNA sequencing to determine the full complement of GC transcriptional states and to address how these states are associated with GC selection. A clear definition of these programs will improve our understanding of how GC B cell selection and affinity maturation are controlled, with implications for vaccine design. Moreover, determining how GC B cell proliferation is controlled will further our understanding of GC-derived B cell lymphomas, which rely on similar mechanisms as GCs to achieve sustained proliferation.